So far, China has made major achievements in narrowing the scope of epidemics in poultry, reducing the number of birds sickened and decreasing the instances of culling, Vice Agriculture Minister Yin Chengjie was quoted as saying on the ministry's Web site.

"But in winter and spring, there is a very high possibility of outbreaks in our country's regions," Yin said. "It's not easy to be optimistic about prevention efforts."

He did not elaborate but health experts have said that the H5N1 virus can survive longer in cold weather and have a bigger chance of infecting poultry - and possibly humans.

Yin also said it was critical to improve bird flu controls in China's south, long the breeding ground for diseases that jump between animals and humans because they often live in close proximity.

Overall improvement in poultry breeding, slaughter and processing is key, Yin said, as are greater efforts to step up immunization before February's Lunar New Year holiday, when chickens and ducks feature prominently in celebration feasts.

Meanwhile, Chinese health officials were continuing their investigation into the case of a 24-year-old man from the eastern city of Nanjing who was the country's 17th bird flu fatality. He died Dec. 2 and his 52-year-old father started showing symptoms a day later and was confirmed to have the disease, health officials have said. They were identified only by their surname, Lu.

Mao Qun'an, spokesman of the Ministry of Health, said human-to-human transmission was possible, although both men could also have been infected by the same source or been infected separately from different sources.

"None of the three possibilities has been confirmed and an in-depth investigation is still being conducted," Mao was quoted as saying by the official Xinhua News Agency.

Hans Troedsson, the World Health Organization's representative in China, said the father was recovering after being treated with the antiviral medicine Tamiflu at the onset of symptoms, a move which "helped the positive outcome of the case."

Eighty-two other people who had contact with the father and son were being monitored for symptoms but so far none appeared to be ill, Troedsson said.

He said the Nanjing cases confirm that "the virus is circulating in the environment, (and) there is a risk that we will see these isolated cases."

But Troedsson said there was no strong indication so far of human-to-human transmission and WHO has not received a request to go to Nanjing to help with the investigation.

"We don't see that is an imminent risk that these cases would be the start of a pandemic," he said.

Xinhua said the son did not have any known contact with dead poultry and the Jiangsu Animal Husbandry and Veterinary Bureau said last week that no bird flu epidemic had been discovered in the province.

Out of the 27 human cases reported in China so far, only one had been forewarned by a poultry outbreak in the vicinity, WHO has said, an indication that the country needs to improve its surveillance systems.

Possible human-to-human transmission of the hard-to-treat H5N1 flu strain has been reported in Hong Kong, Vietnam and Indonesia, but officials determined there was no epidemiological significance because the spread was not sustained.

The Chinese mainland has not confirmed any cases of human-to-human infection, although the sister of a Chinese boy who was diagnosed with H5N1 in 2005 later became sick and died. Authorities were not able to confirm whether the girl had been infected with H5N1.

Scientists have warned that if outbreaks among poultry are not controlled, the virus may mutate into a form more easily passed between people, potentially resulting in millions of deaths. So far, H5N1 has infected 337 people worldwide and killed 207, WHO said.

"The risk of the pandemic has definitely not disappeared," Troedsson said.

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